Your reverse proxy trial didn’t work out perfectly 
(you forgot to mention this in the first post… 
)
I see the url (from your screenshot) that you are using another path (openhab):
http://<Proxy>/openhab/paperui/index.html#/openhab/inbox/search
This is not the “standard” one… 
(http://<IP>:8080/paperui/index.html#/inbox/search)
Press F12 in your Firefox browser to bring up the debug console to see what might be wrong and try to fix (if possible) your Apache reverse proxy configs.
We know that changing the path prefix is a PITA… I wouldn’t use this method…